Precision material handling devices are required in a number of industries. In particular, the handling of semi-conductor wafers at various stages of processing, and the handling of integrated circuit chips after they have been processed from such wafers, requires sophisticated automated handling equipment.
With respect to the handling of semi-conductor wafers, devices are required for moving a wafer from one location through a loadlock (or air lock) to another location, as part of the processing of such wafers in the manufacture of integrated circuit chips. The placement of the wafers throughout the transfer must be precise because of the processing steps which are carried out in the different environments or chambers on opposite sides of the loadlock. Loadlocks or air locks are used to isolate one environment from the other during the processing of the semiconductor wafers.
Because of the nature of the transfer, it is necessary to precisely extend the wafers alternately on two different sides of the loadlock, beyond the loadlock itself, while fully enclosing the wafers and the transfer mechanism within the loadlock as an intermediate step. The use of loadlocks in the processing of semiconductor wafers differs in many respects from larger loadlocks of the type used in space vehicles, manufacturing clean rooms, and the like. The loadlocks which are employed in the processing of semiconductor wafers are used in conjunction with handling equipment for handling very thin, lightweight circular or disk-shaped work pieces having thicknesses in the range of 0.010 to 0.050 inches and diameters measured in inches. Because the processing of the wafers requires them to be free from contamination, the loadlocks, as well as the apparatus used within them, needs to be as free as possible from contaminants and from components which produce contaminants.
The U.S. Pat. No. 4,483,654 to Koch discloses one type of semi-conductor wafer transfer mechanism for transferring wafers from one environment, through a loadlock, to another environment. The mechanism of this patent includes a three-piece gear-driven mechanism. A central drive gear operates through an intermediary or idler gear to rotate a driven gear for effecting rotational positions of lever arms to transport the wafer from a first position in one environment to a second intermediate position within the loadlock, to a third position in another environment on the opposite side of the loadlock chamber. While the mechanism which is disclosed in this patent is capable of relatively rapidly transferring the wafer from one or the other of the two environments to the loadlock and out again, contamination can take place during the isolation vacuum step of operation when the work piece is in the loadlock. This occurs as a result of the wear of the gears involved in the mechanism. Constant wear takes place on these gears, which subjects the interior of the loadlock chamber to minute particle (dust-like) transfers as a result of the wear. In addition, as the gears wear, undesirable play takes place in the transfer mechanism, which cannot be tolerated in the delivery of the wafer to its final processing position on one side or the other of the loadlock.
Another patent for transferring semi-conductor wafers employing a rotary driving source is the U.S. Pat. No. 5,333,986 to Mizukami. The system disclosed in the Mizukami patent inherently has twice the potential for vacuum leaks over the one disclosed in the Koch patent described above. In the Koch system, only a single shaft passes through the loadlock chamber to operate the mechanism located within the chamber. In Mizukami, two shafts extend through the loadlock or vacuum chamber. One of these is the driven rotating shaft (which must be sealed, and the other is an idler rotating shaft (which also must be sealed). Both of these shafts pass through the base or bottom of the chamber. Whenever a seal in a shaft fails, the entire transfer mechanism is down. In the case of Mizukami, there is twice the opportunity for such a failure.
The Mizukami patent employs a parallelogram positioning device utilizing a pair of stainless steel belts to transfer the rotary motion of the input to the different positions of the wafer holding arms. The belt transmission mechanism has low friction between the pulleys and the belts; and there is no gear wear in the system of this patent, as contrasted with the system disclosed in the Koch patent. Typically, however, stainless steel flat belts often involve a large number of strands of wire in the belt in order to obtain the desired flexibility. Internal friction between the strands generates particulate contaminants much in the same way as the wear of the gears described above in conjunction with the Koch patent. The belts of Mizukami must be sufficiently tight to prevent any undesirable play in the overall system which could affect the placement positioning of the wafers and cause a jerky or choppy motion in the wafer positioning arm. If the belts, however, are too tight, internal wear is increased. This is true, either with a flat belt or with a wire rope. Another disadvantage of the apparatus disclosed in Mizukami is that while the system has a fairly extended reach in one direction, in the opposite direction the reach is considerably less.
Another device for handling work pieces is disclosed in the U.S. Pat. No. 4,801,235 to Rauschdorf. This patent employs a rotary drive to produce a linear movement through an arc of travel. The device disclosed in this patent is not a wafer transfer mechanism; so that it does not have a small loadlock chamber of the type typically used with such mechanisms. There also is no linear tracking; and it appears that there is no intention to stop the system operation at some intermediate or "center" position. Rauschdorf does not appear to be concerned with smoothly moving work pieces, but simply moving work pieces from one position to another. With wear at the various pivot points in the Rauschdorf apparatus, the transition through the center position may not be a smooth transition.
Additional U.S. Pat. No. 4,585,045 to Richards; U.S. Pat. No. 4,955,780 to Shimane; and U.S. Pat. No. 5,151,008 to Ishida disclose the uses of belt drives or gear drives in wafer positioning transport apparatus. In the device of the Richards patent, an inherent problem exists through the use of a spring in one of the arms of the transfer mechanism. As the belt wears or stretches, the spring extends the arm to keep the belt tight. This alters placement of the semiconductor wafer in the chamber. As mentioned previously, wafer positioning devices necessarily must be very accurate in the positioning at all stages of operation of the device. Such a wear, which alters placement, is undesirable.
The patent to Shimane employs multiple belts. This involves the potential for much fretting or wear, producing contaminants. In addition, as the belts wear or stretch, they need to be replaced on a regular basis, both in order to maintain the accuracy of the operation of the device, as well as to keep the number of contaminating particulates down within the apparatus. Replacement of belts produces additional maintenance costs and undesirable down time for the system.
The system disclosed in the Ishida patent involves a number of bearing seals, and includes multiple gears which can wear and fret, as described above, to produce contaminants. Again, as discussed previously, when there are more bearing seals in a system, the potential for failure of a seal, and therefore down time for repair of the apparatus, increases directly with the number of seals involved. Another very complex wafer transport mechanism is disclosed in the U.S. Pat. No. 4,728,252 to Lada. The device of this patent has one shaft sealed within another shaft, which rotates independently of the outer shaft. This complex seal mechanism inherently exposes the device to potential failure and fretting. Also, the device employs belts and requires two motors and two motor control circuits, with the attendant wire harness and the like. The complexity of this device makes it expensive; and the problems associated with belt drives located within a loadlock chamber, as described above, also are present with the device of this patent.
It is desirable to provide an accurate wafer transfer mechanism for transferring a wafer or work piece from one environment, through a loadlock, to another environment, which overcomes the disadvantages of the prior art devices discussed above, and which is simple and accurate in its operation.